4io 



MOSSES AND FERNS 



CHAP. 



of an apical cell can be found in the young leaf, and in this 

 respect, as well as the secondary divisions of the stem segments, 

 Azolla differs from Salvinia, where for a long time the young 

 leaves grow, as in most Ferns, by a two-sided apical cell (Fig. 

 238, B). Each leaf lobe in Azolla is divided into an inner small 

 cell and an outer larger one, and the latter is then divided by a 

 radial wall. This formation of alternating tangential and 

 radial walls is repeated with great regularity, and can be traced 



A. 



FIG. 239. Azolla filiculoides. A, Longitudinal section of a dorsal lobe of the leaf, X 

 about 40; n, cavity with colony of Anab&na; h, unicellular hairs; B, epidermis 

 with stomata, Xiso (after Strasburger) ; C, longitudinal section of young root, 

 X22$; sh, root-sheath. 



for a long time. It is not unlike the arrangement of cells fig- 

 ured by Prantl ( (i), PL I, Figs. 2, 3) in some of the Hymeno- 

 phyllacese. 



The fully-developed leaves of Azolla are all alike. In A. 

 filiculoides the two lobes are of nearly equal size, the lower or 

 ventral one, which is submersed, somewhat larger, but simpler 

 in structure. The dorsal lobe shows a large cavity near its base 

 (Fig. 239, A), which opens on the inner side by a small pore. 

 On the outer side the epidermal cells are produced into short 



